Vitamin D

Vitamin D is a fat-soluble vitamin. Fat-soluble vitamins are stored in the body's liver and fatty tissues. Vitamin D acts as both a vitamin and a hormone.

Vitamin D is found in some foods, but the main sources are vitamin D-fortified milk and sunlight. The ultraviolet rays of the sun react with cholesterol present in the skin and create previtamin D3. This compound goes through a series of reactions in the kidneys and the liver, and the final product is vitamin D.

Functions

Vitamin D's functions:

Plays a crucial role in the growth and maintenance of strong, healthy bones

Maintains normal blood levels of calcium and phosphorus

In children with low vitamin D levels, supplementation can improve bone mineral density. While the evidence does not give a clear answer, it has also been suggested that vitamin D supplementation may reduce the risk of
osteoporosis, high blood pressure,
and some forms of cancer.

Vitamin D has also been found to improve pain symptoms in patients with low vitamin D levels.

Recommended Intake

Here are the guidelines for vitamin D intake:

Age GroupRecommended Dietary Allowance or Adequate Intake (IU/Day)

0-12 months400

1-70 years600

71+ years800

Pregnant or nursing women600

IU: international units

The American Academy of Pediatrics recommends supplementation for all children who do not receive at least 400 IU of vitamin D daily. Breastfed babies may require a supplement within the first few days of life. Bottle-fed babies who do not consume enough vitamin-D fortified formula may also need the supplement, as well as any child who does not get plenty of vitamin D in their diet.

As seen above, requirements for pregnant women are the same as for healthy adults, though some believe that pregnant mothers should take more vitamin D than recommended. Furthermore, some experts believe that people at highest risk for vitamin D deficiency, such as older adults or those with limited sun exposure during the winter months, should take 1,000 IU or more daily. However, since the risk of vitamin D toxicity increases with higher doses, such recommendations ought to be discussed individually with a physician.

Vitamin D Deficiency

Rickets—in children, a disease in which the bones become soft and weak

Osteomalacia—in adults, a disease in which the bones become soft and weak

Muscle weakness

More mild vitamin D deficiency is thought to be relatively common, especially in higher latitudes, and may lead to increased risk of osteoporosis.

Vitamin D Toxicity

Since vitamin D is stored in the body and not excreted in the urine like most water-soluble vitamins, it is possible for it to accumulate and reach toxic levels. Here are safe upper level intakes for vitamin D:

Deposits of calcium in soft tissues, like the kidney, heart, and lungs
due to raised levels of calcium in the blood

It is unlikely for sunlight and diet to cause vitamin D toxicity.

If you have problems with any symptoms related to vitamin D toxicity, take less vitamin D supplements.

Major Food Sources

Fortified foods provide the most vitamin D. Examples of foods that may be fortified with vitamin D are:

Milk

Cereal

Orange juice

Yogurt

Margarine

Soy beverages

There are not many foods that are natural sources of vitamin D. Of those foods that have vitamin D naturally are (most to least):

Fatty fish, such as salmon, tuna, and mackerel

Beef liver

Cheese

Egg yolks

Mushrooms

A relatively small amount of sun exposure can provide adequate vitamin D. In a study of naval personnel in submarines, 6 days of sun exposure proved capable of supplying enough vitamin D for 49 sunless days. However, the actual synthesis of vitamin D through sunlight is affected by season, latitude, time of day, cloud cover, smog, use of sunblock, and skin pigmentation.

Health Implications

Populations at Risk for Vitamin D Deficiency

The following populations may be at risk for vitamin D deficiency and may require a supplement:

Breastfed babies—Human milk does not have enough vitamin D. Breastfed babies should receive a 400 IU vitamin supplement each day to make up for this.

Older adults—Studies suggest that adults over age 65 have less ability to synthesize vitamin D through sunlight exposure than adults aged 20-30. They are also likely to spend less time out in the sun.

Locales with limited sun exposure—People who live above latitudes of approximately 40°N and below latitudes of approximately 40°S are at risk for deficiency during most of the winter months.

People with dark skin—Those with darker skin are less able to make vitamin D from the sun.

People who are
obese
—Body fat can bind to some vitamin D preventing it from getting into the blood where it can be used by the body.

People with a reduced ability to absorb dietary fat—Because vitamin D is a fat-soluble vitamin, fat is required for its absorption from foods. Some conditions that can cause fat malabsorption include
Crohn's disease,
cystic fibrosis,
celiac disease, pancreatic enzyme deficiency, and liver disease.

Tips For Increasing Your Vitamin D Intake

Here are tips to help increase your intake of vitamin D:

If you take a vitamin supplement, make sure it contains vitamin D.

Drink vitamin D-fortified milk.

Get sun exposure, but be careful to watch for sunburn. Sunlight is a major cause of
skin cancer.

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.